Electroluminescent device and method



Nov. 13, 1962 D. c. BELL 3,064,155

ELECTROLUMINESCENT DEVICE AND METHOD Filed Sept. 30, 1959 @aired States .Eaten A'3J-)64h55 Patented Nov. 13, 1952 hee nnen sans. sa, 1959, ser. N0. 343,419 16 oiaims. (ci. 31a-10a) This invention relates to electroluminescent lamps and method and, more particularly, to an electroluminescent lamp which is protected against the deleterious effects of moisture-containing atmospheres and voltage overload conditions as well as a method for making such a lamp.

The phenomenon of electroluminescence was first disclosed by G. Destriau, one of his earlier publications appear-ing in London, Edinburgh and Dublin Philosophical Magazine, Series 7, vol. 38, No. 285, pages 700-737 (October 1947). Since this time, electroluminescent devices have been marketed commercially. Electroluminescent lamps normally display a poor maintenance of light output in that the light output decreases as the lamps are operated. This has been disclosed to be due in great part to the deleterious effects of moisture. An electroluminescent lamp can be protected against the deleterious effects of moisture by sealing all moisture-previous portions of the lamp against ingress of moisture by a rigid layer of moisture-impervious material such as epoxy resin. The maintenance of light output for these lamps is quite satisfactor, but if such protected lamps are operated under voltage overload conditions, they are occasionally subject to arcing between the spaced electrodes due to electrical breakdown of the phosphor-dielectric layer. Such an arcing may be progressive in nature and in limited cases, generated gases may tend to fracture the protecting or encasiug epoxy resin layer. If the lamp is sealed with a more flexible moisture-impervious wax for eX- ample, rather than an epoxy resin, arcing usually punctures and may even ignite the waX.

ln order to overcome the foregoing and other difficulties of and objections to prior-art practices, it is the general object of this invention to provide an electroluminescent lamp having improved maintenance of light output as Well as protection against injurious effects of arcing under voltage overload conditions.

it is a further object to provide an electroluminescent lamp wherein the maintenance of light output is improved and tendencies for progressive -arcing in case of electrical breakdown are eliminated.

it is a further object to provide a method for making an electroluminescent lamp which has improved maintenance of light output and wherein tendencies for progressive arcing and fracture of the encasing moisture-im* pervious layer under voltage overload conditions are minimized.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing adjacent to the lmoisture-pervious portions of the lamp, a porous layer principally comprising finely-divided, non-combustible material. A layer of moisture-impervious material seals all moisture-pervious portions of the lamp, including the porous layer, against ingress of moisture. There has also been provided a method for making such a lamp wherein the applied layer of porous material has formed thereover a substantially-continuous skin of plastic material, so that the porositv of the porous layer is maintained during later lampfabrication steps.

For a better understanding of the invention, reference should be had to the accompanying drawings wherein:

FTG. l is sectional elevational view of an electroluminescent lamp wherein phosphor material is embedded in plastic dielectric and wherein moisture-permeable surface portions of the lamp carry thereon a porous layer of tinely-divided, non-combustible material with a moistureimpervious layer thereover;

FIG. 2 is a fragmentary plan View, partly broken away, showing an electroluminescent lamp wherein the lamp electrodes are formed as an interlacing, raster-type grid and wherein moisture-permeable surface portions of the lamp carry thereon a porous layer of finely-divided, noncombustible material with a moisture-impervious layer thereover;

FIG. 3 is a fragmentary sectional elevational view taken on the line III-lll in FIG. 2;

FIG. 4 is a sectional elevational view of an alternative construction for a lamp embodiment generally as shown in FIG. 1, wherein an additional layer of material having a high electrical-puncture strength is also included between the lamp electrodes with a porous layer over moisture-pervious lamp portions and wherein the entire lamp is encased by a layer of moisture-impervious material.

With specific reference to the form of the invention illustrated in the drawings, the numeral 1t) in FIG. l indicates generally an electroluminescent lamp comprising a first light-transmitting electrode layer 12 formed on a glass foundation 14. A layer 16 comprising electrolumi'- nescent phosphor embedded in dielectric material is positioned over the electrode lay-er 12 and a second conducting electrode layer 1S is positioned over the phosphor-dielectric layer 16. The electrode layers 12 and 18, the phosphor-dielectric layer 16 yand the glass foundation 14 form the electroluminescent lamp per se. In the lamp embodiment 10, the electrode 18 is normally deposited on the phosphor-dielectric layer 16 by means of a vacuummetallizing technique, as is conventional. Such a deposited electrode is inherently pervious to penetration by moisture. An additional porous layer 20 principally comprising finelydivided, non-combustible material is carried over the electrode layer 18. A substantially-continuous skin 22 of plastic is carried over the porous layer 20 and a rigid layer 24 of moisture-impervious material, contiguous with the lamp per se, is carried over ythe plastic skin 22 and extends onto the sides 26 of the foundation 14 to form an hermetic seal with the glass. As an eX- ample, the layer 24 is formed of epoxy resin and since the glass foundation 14 and the layer 24 are both quite impervious to penetration by moisture, the resulting electroluminescent lamp is effectively sealed and protected against ingress of moisture, which would otherwise dele teriously effect lamp operation. Light which is generated by the lamp 19 is emitted from the viewing face 28 of the light-transmitting foundation 14.

In fabricating a lamp embodiment 1t? as shown in FIG. l, the light-transmitting foundation 14 first has applied thereover a thin, electrically-conducting layer 12 of lighttransmitting material such as tin oxide and such electrode coatings are conventional. Other similar coatings such as indium oxide can be substituted therefor. An additional strip 39 of light-transmitting electrode material is placed on one side of the foundation 14 to facilitate electrical connection to the lamp 10; Conventional bus bars 32. are axed to the edges of the electrode 12 and to the electrode strip 30 to facilitate electrical connection to the lamp. Conventional lead conductors 34 connect to the bus bars 32 and serve as means for applying an electric potential across the electrode layers 12 and 18. The layer 16 comprising the phosphor-dielectric can be formed of an electroluminescent phosphor embedded in dielectric material, with plastic dielectric preferred for best initial brightness. As an example, the electroluminescent phosphor is formed of finely-divided zinc sulfide which is activated by copper and coactivated by chlorine. Such a phosphor is well known and other electroluminescent phosphors are summarized by Destriau and IveyV 'spaanse article in Proceedings of the LRE., volume 43, No. 12, pages 1911-1940 (December 1955). As` an example, the plastic in which the phosphor is embedded is a lighttransmitting plastic such as polyvinyl chloride acetate. Other suitable plastic materials can be substituted therefor. The relative proportions of phosphor and dielectric which comprises the layer 16 are not critical and by way of example, equal parts by weight of phosphor and dielectric are used to form this layer. The thickness of the layer 16 is two mils and this thickness is subject to considerable variation. After the phosphor-dielectric layer 16 has been formed, the second aluminum electrode layer 18 is vacuum metallized thereon. Other metals such as silver can be substituted for the preferred aluminum. The thickness of the electrode layer 18 is not critical and is subject to variation and as an example is 150()- A.U.

As noted hereinbefore, vacuum-metallized coatings are inherently quite porous in nature and while having sutilcient structural continuity to display electrical continuity through any portion thereof, such coatings also have such discontinuity of structure as to permit ingress of liquid therethrough. Thus in the lamp embodiment 1li as shown in FIG. l, the vacuum-metallized aluminum electrode layer 1S forms a lamp surface which is quite pervious to moisture. in accordance with the present invention, there is placed over the vacuum-metallized aluminum electrode layer 1S, a layer 20 which principally comprises a Vfinely-divided, non-combustible material, such as finely-divided glass or metallic oxide. The preferred finely-divided material is powdered glass and as an example, any of the glass frits disclosed in Vcopending application S.N. 816,405, now Patent 3,005,722, led May 28, 1959, and owned by the present assignee, can be used. Other glasses can be substituted for these indicated glasses. The state of division of the finelydivided glass is subject to considerable variationand as an example, the glass is crushed and passed through a 20G-mesh sieve to remove overly-large particles. ln applying the glass layer in porous form, the lamp is positioued with the moisture-pervious aluminum electrode layer 18 substantially horizontal and facing upward. There is applied over this horizontally-disposed aluminum electrode layer 1S, a predetermined thickness of a layer of slurry ywhich comprises a mixture of the timely-divided glass and plastic-solvent solution. The plasticsolvent solution of the slurry can be formed of any plastic material and solvent therefor which will not Ireact with the glass or aluminum and which has a density which is less than that of the finely-divided glass, so that a substantial portion of the nely-divided glass will settle to the bottom of the slurry layer after it is applied. As an example, the preferred plastic which is used in forming the slurry layer is a polyamide resin such as sold under the generic term Nylon. Alcohols are solvents for Nylon and ethyl alcohol in the form of denatured alcohol is preferred, althoughV other suitable solvents can be used. The preferred Nylon-solvent solution is 5% by weight Nylon and 95% by weight ethanol. When forming the porous Y layer 2t? which principally comprises the r'inelydivided glass for example, a small portion of residual Nylon will remain throughout the porous layer Ztl and will serve to effect some bonding between the porous layer 26 and the electrode layer 1S, although the layer 2t) will still display considerable porosity. By way of furl ther example, the finely-divided glass can be mixed in as one hour for example. Thereafter the solvent is evaporated by heating the lamp to a temperature of approximately 60 C. for approximately one hour. This will form a substantially-continuous skin 22 of plastic over the porous layer 2li. The resulting porous layer 2% will have a predetermined thickness of approximately 50 mils and the substantially-continuous skin 22 of plastic which is formed over and bonded to the porous layer Ztl will have a predetermined thickness of approximately 2 to 3 mils. Thereafter the layer 24 of moisture-impervious material such as epoxy resin is applied in predetermined thickness over the plastic skin 22 so as to extend down over the sides 26 of foundation 14 to form an hermetic seal. The epoxy resin is applied in unpolymerized liquid form as generally disclosed in copending application SN. 822,231, lied June 23, 1959. and owned by the present assignee. Also, as is pointed out in this lastmentioned copending application, other additives can be used to supplement the epoxy resin in order to lend llexibility thereto. As an example, the layer 24 has a thickness of 45 mils and this thickness is subject to considerable variation. The preferred catalyst for the epoxy resin is about 10% by weight of the resin of a mixture of 50% by weight diethylenetriamine and 50% by weight acrylonitrile. Such catalysts for use with epoxy resins are well known. After the resin has been applied to the indicated thickness, the lamp is heated to a temperature of 50 C. for thirty minutes. This causes the epoxy Aresin to polymerize to form a rigid, moisture-impervious layer of predetermined thickness. The substantially-continuous skin 2f. of plastic over the porous layer 2li prevents the epoxy -resin when in liquid and unpolymerized form penetrating and filling the voids which are contained between the individual particles of finely-divided material which comprise the porous layer 20.

While the foregoing example has been carried through in detail, it should be understood that other plastic materials and solvents can be substituted for the preferred Nylon-alcohol solution. As indicated, the thickness of the porous layerZt is subject to considerable variation as is the state of division of the finely-divided, non-combustible material which comprises this porous layer, provided that voids are maintained between the individual particles which comprise the layer 2G, The thickness of the intermediate plastic skin 22 is also subject to considerable variation. A small amount of unsettled finelydivided, non-combustible material may remain embedded in the formed plastic skin 22 and this will not impairV layer 2li, which acts to bulfer the physical and thermal shocks ofV such breakdown. Thus the formed arc quenches immediately, resulting only in a small pinhole in the aluminum electrode 18 at the point of arc. The light-diffusing characteristics of the phosphor-dielectric layer serve to mask any reduced light. emission in the area of such formed pinholes. in lamp constructions wherein the porous layer 2li is dispensed with, arcing due to voltage overload melts the aluminum electrode and the arc is apt to become progressive at this point since the melted aluminum is not rapidly dissipated to quench the arc. ln addition, the gases which are generated may cause the rigid epoxy resin layer 2-3 to crack, thereby impair-ing the continuity of this moisture-impervious layer. Lamps constructed in accordance with the present invention were repreatedly subiected to voltage overloadconditions, wherein the voltage was applied as pulses which exceeded by ten times the rated lamp voltage. While this caused incipient breakdowns between the lamp electrodes, such breakdowns were self-healing and were not injurious to the performance or appearance of the lamp.

In the preferred lamp construction, the moisture-impervious layer 24 is formed of rigid material such as epoxy resin. This rigid layer imparts some structural stability to the lamp and facilitates handling. The epoxy resin can be replaced by a more-flexible, moisture-irnpervious material such as beeswax, applying same in melted form. When using a material such as beeswax, the porous layer 20 is beneficial in that any arcing will not puncture or ignite the layer of beeswax, because of the buffering and cushioning action of the layer 20. Other suitable moisture-impervious waxes or other moisture-impervious materials can be substituted for the beeswax or the preferred epoxy resin. Whether beeswax or epoxy resin or other suitable material is used in forming the layer 24, the substantially-continuous plastic skin 22 will maintain the -porosity of the layer 20 when the layer 24 is applied thereover.

In FIGS. 2 and 3 are shown an alternative lamp construction lltla wherein the lamp electrodes 36 are formed as an interlacing, raster-type grid mesh. Such an electrode arrangement is generally described in U.S. Patent No. 2,684,450, dated Iuly 20, 1954. In accordance with the present invention, the porous layer 20a as previously described is formed over the phosphor-dielectric layer 16a which is contained between the electrodes 36. A skin 22a of plastic as previously described is formed over the porous layer 20a and the layer 24a of moisture-impervious material is formed over the plastic skin 22a to seal the lamp. Such a construction will inhibit any tendency for cracking or failure of the layer 24a it electrical breakdown occurs between the cell electrodes because of voltage overload conditions. If the foundation 14a is not light transmitting, the layers 20a, 22a and 24a are selected to be light transmitting.

In FIG. 4 is shown a further lamp embodiment 10b 'which generally corresponds to the embodiment 16 as shown in FIG. 1 except that an additional layer 38 of material having a very high dielectric constant and high electrical puncture strength is also included between the lamp electrodes. Such an additional layer can be formed of barium titanate or titania for example, as disclosed in British Patent No. 765,076, published January 2, 1957. The lamp embodiment as shown in FIG. 4 is also modified somewhat in that the'layer '24h which comprises light-transmitting epoxy resin for example encapsulates the entire lamp. Such a construction lends itself to a dip process for applying the moisture-pervious layer Zlib.

The material which principally comprises the porous layer for any of the embodiments as illustrated and described hereinbefore preferably is the indicated inelydivided glass. Any nely-divided, non-combustible material which will not react with the aluminum lamp electrode can be substituted for the finely-divided glass. As examples, oxides of magnesium, aluminum, silicon or mixtures thereof can be substituted for the glass materials. Other suitable timely-divided, non-combustible materials are silicates, phosphates, stannates or zirconates for example. As a further possible alternative embodiment, the porous layer as described hereinbefore can be formed of a desiccant such as aluminum oxide. With such a construction, the embedding dielectric can be selected to withstand relatively-high temperatures and the lamp can be heated to 180 C. for example before the rigid, moisture-impervious layer is formed thereabout in order to activate the desiccant. Such a lamp will display excellent maintenance of light output.

As a possible alternative construction in any of the foregoing embodiments, the plastic dielectric material which is used as an embedding medium for the phosphor can be replaced by glass dielectric. As another possible alternative construction, the glass foundation for the lamp embodiment 10 as shown in FIG. l can be replaced by a plastic foundation and a light transmitting copper iodide electrode carrier thereon. In accordance with the present invention, the porous layer, intermediate skin of plastic and moisture-impervious layer will inhibit ingress of moisture to the operative portions of these lamps.

While best-known embodiments have been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

I claim:

l. In combination with an electroluminescent lamp having moisture-pervious portions, a moisture-impervious layer contiguous with and encasing all moisture-pervious portions of said lamp, and a porous layer principally comprising finely-divided non-combustible material between said lamp and said moisture-impervious layer.

2. An velectroluminescent lamp comprising, spaced electrodes, material comprising electroluminescent phosphor included between said spaced electrodes, and means for applying an electric potential across said spaced electrodes, a moisture-impervious layer contiguous with and encasing all moisture-pervious portions of said lamp, and a porous layer principally comprising finely-divided non-combustible material included between said moistureimpervious layer and said lamp.

3. An electroluminescent lamp comprising, spaced electrodes, material comprising electroluminescent phosphor included between said spaced electrodes, and means for applying an electric potential across said spaced electrodes, a porous layer principally comprising finely-divided non-combustible material over moisture-pervious portions of said lamp, a substantially continuous plastic layer over said porous layer, and a moisture-impervious Vlayer over said plastic layer and encasing al1 moisturepervious portions of said lamp.

4. An electroluminescent lamp comprising, spaced electrodes, material comprising electroluminescent phosphor included between said spaced electrodes, and means for app-lying an electric potential across said spaced electrodes, a porous layer principally comprising finely-divided glass bonded to moisture-pervious portions of said lamp, a substantially continuous polyamide resin layer bonded to said porous layer, and a rigid moisture-impervious epoxy resin layer bonded to said polyamide resin layer and encasing all moisture-pervious portions of said lamp.

5. An electroluminescent lamp comprising, spaced electrodes, material comprising electroluminescent phosphor included between said spaced electrodes, and means for applying an electric potential across said spaced electrodes, a porous layer of predetermined thickness and principally comprising finely-divided glass bonded to moisture-pervious portions of said lamp, a substantiallycontinuous layer of predetermined thickness and principally comprising polyamide resin bonded to said porous layer, and a rigid moisture-impervious layer of predetermined thickness and principally comprising epoxy resin bonded to said polyamide resin layer and encasing all moisture-pervious portions of said lamp.

6. An electroluminescent lamp comprising, a glass foundation, a first light-transmitting electrode layer carried thereon, material comprising electroluminescent phosphor over said rst electrode layer, a second moisture-pervious electrode layer over said material comprising electroluminescent phosphor, and means for applying an electric potential across said irst and second electrode layers, a porous layer principally comprising finely-divided non-combustible material over said second electrode layer, a substantially-continuous layer prinycipally comprising plastic material over said porous layer,

and a moisture-impervious layer over said plastic layer and sealed to said glass foundation.

'Z 7. An electroluminescent lamp comprising, a glass foundation, a first light-transmitting electrode layer carried thereon, materal comprising electroluminescent phosphor over said iirst electrode layer, a second moisture-pervious electrode layer over said material comprising7 electroluminescent phosphor, and means for applying an electric potential across said tirst and second electrode layers, a porous layer of predetermined thickness and principally comprising finely-divided non-combustible material over said second electrode layer, a substantially-continuous plastic layer of predetermined thickness over said porous layer, and a layer of rigid moisture-impervious material of predetermined thickness over said plastic layer and sealed to said glass foundation.

8. An electroluminescent lamp comprising, a glass foundation, a first light-transmitting electrode layer carried thereon, material comprising electroluminescent phosphor over said first electrode layera second moisture-pervious aluminum electrode layer over said material comprising electroluminescent phosphor, and means for applying an electric potential across said first and second electrode` layers, a porous layerrprincipally comprising iinely-divided glassvover said second electrode layer, a substantially-continuous layer principally comprising plastic material over said porous layer, and a rigid moisture-impervious layer principally comprising epoxy resin over said plastic layer and sealed to said glass foundation.

9. An electroluminescent lamp comprising, a glass foundation, aL first light-transmitting electrode layer carried thereon, material comprising electroluminescent L phosphor over said"first electrode layer, a second moisture-pervious aluminum electrode layer over said mate'rial'V comprising electroluminescent phosphor, and

, means for applying an electric potential across said rst and second electrode layers, a porous layer of predetermined thickness and principally comprising finely-divided glass over said second electrode layer, a substantiallycontinuous layer of predetermined thickness and principally comprising polyamide resin over said porous layer, and a rigid moisture-impervious layer of predetermined thickness` and principally comprising epoxy resin over said plastic layer and sealed to said glass foundation.

l0. An electroluminescent lamp comprising, a glass foundation, a first light-transmitting electrode layer carred thereon, material comprising electroluminescent phosphor over said first electrode layer, a second moisture-pervious 'electrode layer over said material comprising electroluminescent phosphor, and means for applying an electric potential across said first and second electro-de layers, a porous layer of predetermined thickness and principally comprising finely-divided desiccatng material over said Vsecond electrode layer, a substantiallycontinuous plastic layer of predetermined thickness over said porous layer, and a moisture-impervious layer of predetermined thickness oversaid plastic layer and sealed to said glass' foundation.

ll. An electroluminescent lamp comprising, a glass foundation, a first light-transmitting electrode layer carried thereon, material comprising electroluminescent phosphor over said iirst electrode layer, a second moisture-pervious aluminum electrode layer oversaid material comprising electroluminescent phosphor, and means for applying an electric potential across said first and second electrode layers, a porous layer of predetermined thickness a-nd principally .comprising finely-divided glass bonded to and over said second electrode layer, a substantially-continuous layer of predetermined thickness and principally comprising nylon bonded to and over said porousV layer, and a rigid moisture-impervious layer ofV predetermined thickness and principally comprising Vepoxy resin bonded to and over said nylon layer and sealed to said glass foundation.

l2. The method of encasmg moisture-pervrous portions of an electroluminescent lamp with a layer principally comprising moisture-impervious material, while lli o ci still maintaining a porous layer principally comprising linely-divided non-combustible material between moisture-pervious portions of Vsaid lampand the encasing layerLwhichmethod comprises-placing said lamp with moisture-pervious portions substantially vhorizontal and facing upward; applying over the horizontally-disposed, moisture-pervious lamp portions a predetermined amount of a slurry layer formed of a mixture of nely-divided non-combustible material and plastic-solvent solution, with the density of the finely-divided material greater than the density of the plastic-solvent solution; allowing a substantial 'portion of the mixed finely-divided material to settle to the bottom of the applied Yslurry layer to form a concentration of plastic at the upper surface of the applied slurry layer; evaporating solvent from the applied slurry layer to form a substantially-continuous plastic skin over the settled finely-divided material; applying over the formed plastic skin a liquid layer of material which when in solid form is impervious to moisture; and solidifying the applied layer of liquid material.

13. The method of encasing moisture-pervious portions of an electroluminescent lamp, which include a moisturepervious lamp electrode, with a rigid layer principally comprising moisture-impervious plastic, While still maintaining a porous layer principally comprising finely-divided non-combustible material between the moisture-pervious lamp electrode and the rigid encasing layer, which method comprises: placing said lamp with the moisturepervious lamp electrode substantially horizontal and facing upward; applying over the horizontally-disposed, moisture-pervious lamp electrode a predetermined amount of a slurry layer formed of a mixture of finely-divided noncombustible material and plastic-solvent solution, with the density of the finely-divided material greater than the density of the plastic-solvent solution; allowing a substantial portion of the mixed finely-divided material to settle to the bottom of the applied slurry layer to forma concentration of elastic at the uriner surface of the applied slurry layer; evaporating solvent from the applied slurry layer to form a substantially continuous plastic skin over the settled finely-divided material; applying over the formed plastic skin a liquid layer principally comprising unpolymerized Vthermosetting resin which is rigid and miosture-impervious when polymerized; and polymerizing the. applied layer principally comprising thermosetting resin.

14. The method of encasing moisture-pervious portions of an electroluminescent lamp, which include a moisturepervious lamp electrode, with a rigid layer principally comprising moisture-impervious plastic, while still maintaining a porous layer principally comprising iinely-divided glass between the moisture-pervious lamp electrode and the rigid encasinrr laver, which method comprises: placing said lamp with the moisture-pervious lamp electrode substantially horizontal and facing upward; applying over the horizontally-disposed, moisture-pervious lamp electrode a predetermined amount of a slurry layer formed of a mixture of finely-divided glass and plastic-solvent solution, with the density of theV finely-divided glass greater than the density of the plastic-solvent solution;

allowinga substantial portion of the mixed finely-divided glass to settle to the bottom ot' the applied Vslurry layer to form a concentration of plastic at the upper surface of Ythe applied slurry layer; evaporating solvent from the applied slurry layer to form a substantially continuous plastic skin over the settled finely-divided glass; applying over the formed plastic skin a liquid laver principally comprising unpolymerized thermosetting resin which is rigid and moisture-impervious when polymerized; and

polymerizing'the applied layer principally comprising while still maintaining a porous layer principally comprising nely-divided glass between the moisture-pervious lamp electrode and the rigid encasing layer, which method comprises: placing said lamp with the moisturepervious lamp electrode substantially horizontal and facing upward; applying over the horizontally-disposed, moisture-pervious lamp electrode a predetermined amount of a slurry layer formed of a mixture of finely-divided glass and Nylon-alcohol solution, with the density of the finely-divided glass greater than the density of the Nylonalcohol solution; allowing a substantial portion of the mixed nely-divided glass to settle to the bottom of the applied slurry layer to form a concentration of Nylon at the upper surface of the applied slurry layer; evaporating alcohol solvent from the applied slurry layer to form a substantially continuous Nylon skin over the settled nely-divided glass; applying over the formed Nylon skin a liquid layer principally comprising unpolymerized thermosetting epoxy resin, and polymerizing the applied layer principally comprising thermosetting epoxy resin.

16. The method of encasing moisture-pervious portions of an electroluminescent lamp, which include a moisturepervious aluminum lamp electrode, with a rigid layer principally comprising moisture-impervious epoxy resin, while still maintaining a porous layer principally comprising finely-divided glass between the moisture-pervious lamp electrode and the rigid encasing layer, which method comprises: placing said lamp with the moisture-pervious lamp electrode substantially horizontal and facing up- Ward; applying over the horizontally-disposed, moisturepervious lamp electrode a predetermined amount of a slurry layer formed of a mixture of about by volume of finely-divided glass and 30% by volume of plasticsolvent solution consisting of 5% by weight Nylon and by weight ethanol, allowing a substantial portion of the mixed finely-divided glass to settle to the bottom of the applied slurry layer to form a concentration of Nylon at the upper surface of the applied slurry layer; evaporating alcohol solvent from the applied slurry layer to form a substantially continuous plastic skin over the settled nely-divided glass; applying over the formed plastic skin a liquid layer principally comprising unpolymerized thermosetting epoxy resin; and polymerizing the applied layer principally comprising thermosetting epoxy resin.

References Cited in the lile of this patent UNITED STATES PATENTS 2,183,256 Gabler Dec. 12, 1939 2,566,349 Mager Sept. 9, 1951 2,755,406 Burns July 17, 1956 2,821,646 Walker Jan. 28, 1958 2,901,652 yFridrich Aug. 25, 1959 2,918,594 Fridrich Dec. 22, 1959 FOREIGN PATENTS 787,401 Great Britain Dec. l1, 1957 

